Among the important advances in receptor biology in the past decade is the recognition of a large family of inhibitory receptors (IRS) that function to limit cellular responses to activating ligands. These receptors serve a variety of functions ranging from prevention of NK cell attack on self, to limitation of arthritic and allergic inflammation, and termination of antibody responses. IRS receptors offer a novel approach for therapeutic intervention using receptor agonists or stimulators of downstream signaling pathways. Our goal is to explore this possibility using mast cells in the context of a mast cell-dependent model of rheumatoid arthritis (RA). Mast cells express multiple inhibitory receptors that utilize one of two primary effectors; the S_H2-containing phosphotyrosine pjiosphatase SHP1, or the SH2-containing inositol 5- pjiosphatase SHIP1. Receptors that utilize SHIP are better "targets" for therapy because, unlike SHP1, SHIP can function in trans to inhibit signaling by remotely ligated receptors. Studies in Aim 2 will focus on molecular characterization and targeting of novel SHIP1-coupled mast cell receptors. Aim 1 will address the mechanism of SHIP1 regulation of mast cell function in vivo in RA, focusing on its ability to degrade Ptdlns3,4,5P3, act as an adaptor to stimulate rasGAP and generate Ptdln3,4P2. In Aim 3 we will address the molecular mechanism by which SHIP1 inhibits signaling in trans. Studies will utilize biochemical and cell biological approaches in conjunction with rat (RBL2H3) and human (LAD2) mast cell leukemias, and bone marrow derived mast cells (BMMC). We will also utilize adoptive reconstitution of mast cell deficient W/W mice with WT and genetically manipulated BMMC followed by analysis of the anti-GPI induced arthritic response to test the in vivo function of receptors and effectors in RA. Relevance - Allergy and rheumatoid arthritis are among the most widespread and disabling clinical conditions. Inflammation associated with each of these pathologies depends on activation of mast cells. In this proposal we request funds to test a novel approach to treatment of these conditions. The basis of this approach is stimulation of receptors and signaling pathways that function normally to limit mast cell activation, in hopes of increasing their inhibition of mast cell activation. This approach may be applicable to other inflammatory conditions involving cells that express inhibitory receptors.